There has been a need for a magnetic-field detecting device which can detect a magnetic field such as a magnetic signal (magnetic field) generated by living body cells, with high accuracy, for example, with a high degree of resolution. In an attempt to meet this need, there has been proposed a magneto-impedance sensor utilizing an MI (Magneto-Impedance) effect of an amorphous wire (amorphous metal alloy in the form of a wire) wherein an impedance of the amorphous wire being energized with a high-frequency electric current changes in the presence of an external magnetic field. Patent Document 1 discloses an example of such a magneto-impedance sensor. This magneto-impedance sensor is capable of a minute magnetic field as a change of the impedance of the above-indicated amorphous wire, making it possible to detect the magnetic field with a high degree of resolution.
For measuring a subject which generates a minute magnetic field, it is necessary to reduce an influence of the external magnetic field such as the terrestrial magnetism, as well to use the sensor having a high degree of measuring resolution as described above. To reduce the influence of the external magnetic field, it is known to conduct a differential operation of outputs of a plurality of sensors. Namely, one of the sensors is configured to detect both of a magnetic field generated by the subject and the external magnetic field, while the other sensor is configured to detect only the external magnetic field, so that only the magnetic field generated by the above-indicated subject can be obtained on the basis of a difference between the outputs of the two sensors.